UNL Plant Pathologist Dr. Tamra Jackson-Ziems and her team recently confirmed tar spot in Richardson County, just north of Falls city. The disease infestation is in very low levels, but it is important to begin scouting your corn field. The Corn ipmPIPE helps track the appearance of tar spot. It is available online at corn.ipmpipe.org/tarspot. When you look at the map, keep in mind that the gray color signifies counties where tar spot was found in previous years and the highlighted yellow ones indicate that a sample from one or more fields in that county tested positive this year. Last year, tar spot was confirmed in 47 counties in eastern Nebraska.
What does Tar Spot look like?
Tar spots produce small, raised black circular spots that look like tar. These spots cannot be scraped off the leaf and often have a tan to brown halo around them, giving them a “fisheye” appearance. These black spots can be mistaken for a few other things:
- Insect frass: This can be rubbed off with some moisture and does not show through both sides of the leaf like tar spot.
- Saprophytic fungi: These fungi grow on dead brown tissue and thrive in moist conditions. These saprophytic fungi are not raised above the surface like tar spot and may not have clearly defined margins as well.
- Rust spores: At the end of season, orange-red spores in common and southern rust turn black and looks similar to tar spot. Rust spores can be rubbed off and often leave an orange to black mark on your finger.
Because of these similarities, it is important to be extra careful while scouting your corn field for tar spot.
What causes Tar Spot and where to scout?
Moderate temperatures and frequent early and mid-season rainfall with average leaf wetness of 7 hours or more create ideal conditions for disease. With all the rain we have been getting this year we need to be especially cautious. The disease could be observed as early as V8 corn stage. The fungus overwinters in crop debris so the disease will reappear in the same areas. Once the fungus is established and the weather conditions become favorable again, it can reproduce and cause infections.
At this time of the season, tar spot is expected to be found in the lower part of the canopy because it is a residue-borne disease. If you have a center pivot, inner spans may be more conducive for disease than the outer irrigation span and that is where you will need to scout first.
What should I do if I find Tar Spot?
Contact your local extension educator if you see anything that fits the description of tar spot. Samples with suspected tar spot can be sent to the UNL Plant and Pest Diagnostic Clinic (plantpathology.unl.edu/plant-pest-diagnostic-clinic) and they will diagnose the samples free of charge.
Once tar spot is confirmed, foliar fungicide applications should be considered. You can visit https://go.unl.edu/2023cornfungicideefficacy for specific products and their ratings for tar spot and other corn diseases.
By Ritika Lamichhane – rlamichhane3@unl.edu
Extension Educator in Nemaha County
402-274-4755